- You buy candy that you like so if you get no trick-or-treaters, you have a little treat for yourself, or
- You buy candy that you don't like so you aren't tempted to eat it all.
I used to love sitting in those WW meetings, listening to all the old ladies bragging on how they bought Crappy Cardboard or Icky Bars so that they wouldn't be tempted to dig in before the kids, and all I kept thinking was "Yeah, but the kids are just throwing that crap out". I may have even said it one or two times--you know me.
And to those weight-conscience folks who give out pencils, stickers, or pennies--that stuff gets tossed too. Kids want candy, folks--and the good kind. If you are in any doubt on what the "good" kind is--ask any kid in the store, they'll tell you.
Or you can ask me. I happen to have many years of experience digging through my kids' Halloween bucket.